A. Field of the Invention
The present invention relates to a superjunction semiconductor device applicable to a MOSFET (insulated gate field effect transistor), IGBT (insulated gate bipolar transistor), bipolar transistor, or the like, with a high breakdown voltage and large current capacity appropriate for a resin mold seal structure.
B. Description of the Related Art
Generally, a vertical power semiconductor device has a structure in which an on-state current flows in a vertical direction between the main surfaces of a semiconductor substrate, and a depletion layer caused by a reverse bias voltage applied to a main junction when turning off extends in the vertical direction between the main surfaces. In order to obtain desired breakdown voltage characteristics in the vertical power semiconductor device, firstly, it is necessary to design a drift layer to a layer resistance and layer thickness commensurate with the desired breakdown voltage in order to prevent the critical electrical field strength of the silicon semiconductor device being reached, and breakdown occurring, at a breakdown voltage lower than the desired breakdown voltage. However, the higher the breakdown voltage becomes, the higher the resistance and the greater the thickness needed for the drift layer, meaning that it is normally unavoidable that the voltage drop (on-resistance) caused by the on-state current also increases. That is, in a vertical power semiconductor device, the breakdown voltage characteristics and voltage drop (on-resistance) characteristics are normally conflicting characteristics from the point of view of element structure design, and it is generally considered difficult to achieve a structure wherein both are simultaneously improved. The kind of relationship between the breakdown voltage characteristics and voltage drop characteristics described above relating to structure design of a vertical power semiconductor device is sometimes called a trade-off relationship.
A superjunction semiconductor device is known to be a semiconductor device in which it is possible to eliminate this kind of trade-off relationship and simultaneously improve both characteristics. The superjunction semiconductor device structure includes a parallel pn layer having plural n-type drift regions, with an impurity concentration (layer resistance) higher than an impurity concentration commensurate with the heretofore known design breakdown voltage, and p-type partition regions are alternately repetitively disposed in a vertical direction with respect to the main surfaces in the drift layer. It also has plural p-type junctions vertical with respect to the main surfaces (for example, refer to U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215, and JP-A-9-266311). With the superjunction semiconductor device, even when the drift layer impurity concentration is higher than the impurity concentration envisaged for the design breakdown voltage, when the depletion layer spreads from the pn junction between each region in the parallel pn structure when turning off, each region of the parallel pn structure is of a width small enough to be completely depleted at a low withstand voltage, meaning that it is possible to simultaneously achieve a low voltage drop (low on-resistance) and an increased breakdown voltage.
Meanwhile, in order for a vertical power semiconductor device to be a semiconductor element with a high breakdown voltage and high reliability, a breakdown voltage structure commensurate with a high breakdown voltage is needed in an element peripheral portion. This kind of breakdown voltage structure includes a structure, provided in a peripheral portion encircling an element active portion touching a main current path of the element, which has an electrical field concentration reduction function and charge resistance. The electrical field concentration reduction function is a function that reduces the electrical field concentration which is liable to occur at the drift layer terminal when applying an off-state voltage, thus preventing a low breakdown voltage breakdown. Charge resistance is a function that prevents a drop in breakdown voltage reliability, wherein a charge applied to the surface affects the extension of the depletion layer below the surface, and the breakdown voltage decreases along with the passing of time.
One example of a semiconductor device including a structure that ensures this kind of trade-off relationship elimination and a guarantee of long-term breakdown voltage reliability is already known. This semiconductor device firstly, in order to eliminate the trade-off relationship, includes a superjunction structure having the previously described parallel pn layer in the element active portion of the drift layer. Furthermore, the semiconductor device includes, in a peripheral portion of the element active portion, an element peripheral portion having a parallel pn layer of a lattice form plane pattern with a repeating pitch smaller than the pitch of the parallel pn layer of the element active portion. Further still, the semiconductor device is a superjunction semiconductor device having a structure wherein an n− region of a uniform concentration lower than the concentration of the parallel pn layer covers the surface of the lattice form parallel pn layer of the element peripheral portion. According to the superjunction semiconductor device, as it is possible to realize an element with low on-resistance and high breakdown voltage, and to prevent an overspreading of the depletion layer caused by a surface charge, it is possible to achieve an improvement in charge resistance (WO2011/013379).
With the superjunction semiconductor device described in WO2011/013379, however, although breakdown voltage charge resistance is ensured at a level of surface charge amount applied to the surface of the element peripheral portion in a range of Qss=±1.0×1012 cm−2, there is a danger of a drop in breakdown voltage in the case of a resin mold seal that has an impurity ion concentration higher than the above level. That is, a charge resistance at a surface charge amount level of Qss=±1.0×1012 cm−2 is insufficient for the superjunction semiconductor device to be a device of a resin mold seal structure. In order for the superjunction semiconductor device to be a resin mold seal structure superjunction semiconductor device that suppresses a drop in breakdown voltage and has high reliability, it is necessary to further improve the charge resistance.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.